[libc++] tests with picolibc: prevent looking for unneeded "rt" library (#82262)

Picolibc does not provide the clock_gettime function nor the "rt" library.
check_library_exists was invalidly detecting the "rt" library due to cmake issue 
present, when cross-compiling[1]. This resulted with "chrono.cpp" trying to link
to the "rt" library and following error:
    unable to find library from dependent library specifier: rt

[1] https://gitlab.kitware.com/cmake/cmake/-/issues/18121

Author: domin144

libcxx/cmake/config-ix.cmake

@@ -1,5 +1,6 @@
 include(CMakePushCheckState)
 include(CheckLibraryExists)
+include(CheckSymbolExists)
 include(LLVMCheckCompilerLinkerFlag)
 include(CheckCCompilerFlag)
 include(CheckCXXCompilerFlag)
@@ -97,6 +98,8 @@ int main(void) { return 0; }
   cmake_pop_check_state()
 endif()
 
+check_symbol_exists(__PICOLIBC__ "string.h" PICOLIBC)
+
 # Check libraries
 if(WIN32 AND NOT MINGW)
   # TODO(compnerd) do we want to support an emulation layer that allows for the
@@ -116,6 +119,10 @@ elseif(ANDROID)
   set(LIBCXX_HAS_PTHREAD_LIB NO)
   set(LIBCXX_HAS_RT_LIB NO)
   set(LIBCXX_HAS_ATOMIC_LIB NO)
+elseif(PICOLIBC)
+  set(LIBCXX_HAS_PTHREAD_LIB NO)
+  set(LIBCXX_HAS_RT_LIB NO)
+  set(LIBCXX_HAS_ATOMIC_LIB NO)
 else()
   check_library_exists(pthread pthread_create "" LIBCXX_HAS_PTHREAD_LIB)
   check_library_exists(rt clock_gettime "" LIBCXX_HAS_RT_LIB)

libcxx/test/std/time/time.clock/time.clock.file/to_from_sys.pass.cpp

@@ -10,9 +10,6 @@
 
 // UNSUPPORTED: availability-filesystem-missing
 
-// "unable to find library from dependent library specifier: rt"
-// XFAIL: LIBCXX-PICOLIBC-FIXME
-
 // <chrono>
 //
 // file_clock

libcxx/test/std/time/time.clock/time.clock.hires/now.pass.cpp

@@ -6,9 +6,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-// "unable to find library from dependent library specifier: rt"
-// XFAIL: LIBCXX-PICOLIBC-FIXME
-
 // <chrono>
 
 // high_resolution_clock

libcxx/test/std/time/time.clock/time.clock.system/from_time_t.pass.cpp

@@ -6,9 +6,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-// "unable to find library from dependent library specifier: rt"
-// XFAIL: LIBCXX-PICOLIBC-FIXME
-
 // <chrono>
 
 // system_clock

libcxx/test/std/time/time.clock/time.clock.system/now.pass.cpp

@@ -6,9 +6,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-// "unable to find library from dependent library specifier: rt"
-// XFAIL: LIBCXX-PICOLIBC-FIXME
-
 // <chrono>
 
 // system_clock

libcxx/test/std/time/time.clock/time.clock.system/to_time_t.pass.cpp

@@ -6,9 +6,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-// "unable to find library from dependent library specifier: rt"
-// XFAIL: LIBCXX-PICOLIBC-FIXME
-
 // <chrono>
 
 // system_clock

libcxx/test/std/time/time.point/time.point.nonmember/op_-duration.pass.cpp

@@ -6,9 +6,6 @@
 //
 //===----------------------------------------------------------------------===//
 
-// "unable to find library from dependent library specifier: rt"
-// XFAIL: LIBCXX-PICOLIBC-FIXME
-
 // <chrono>
 
 // time_point